Brpnsted sites

A wide variety of NMR methods are being applied to understand solid acids including zeolites and metal halides. Proton NMR is useful for characterizing Brpnsted sites in zeolites. Many nuclei are suitable for the study of probe molecules adsorbed directly or formed in situ as either intermediates or products. Adsorbates on metal halide powders display a rich carbenium ion chemistry. The interpretation of NMR experiments on solid acids has been greatly improved by Ae integration of theoretical chemistry and experiment. 

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

The detection of Brpnsted acid sites, SiO(H)Al, is the most recent achievement of 170 NMR of zeolites [119-121]. High magnetic fields and double resonance techniques have allowed the observation of this important species in zeolite HY [120]. Chemical shifts of 21 and 24 ppm have been reported for zeolite HY for the Brpnsted sites in the supercage and sodalite cage, respectively [119]. Quadrupole interaction parameters are Cq = 6.0 and 6.2 MHz and r] = 1.0 and 0.9, respectively. Signal enhancement by 1H-170 cross-polarization has also permitted the detection of the acid sites in zeolite ZSM-5 [119], where they exist with lower abundance than in HY. 

Infrared spectra of pyridine adsorbed on dehydrated TS-1 and Ti-MCM-41 of comparable Ti content indicated the presence of only Lewis acid sites (Fig. 13). The infrared absorptions at 1595 and 1445 cm-1 are attributed to hydrogen-bonded pyridine (Si/Ti-OH—pyridine) and those at 1580 and 1485 cm-1 to pyridine bonded to weak Lewis acid sites (Fig. 12). Brpnsted sites, if present,... 

Since ITQ-4/SSZ-42/MCM-58 have been prepared as aluminosilicates with Si/Al ratios of 20 to °°, which possess Brpnsted sites, there is potential for acid catalysis. Some preliminary accounts of catalytic cracking, hydrocracking, dewaxing, alkylation, hydroisomerization, and reforming reactions have been reported (47, 62-64). 

Brpnsted acid sites can be further dehydroxylated to form Lewis acid sites as shown also in Figure 13.2. The elimination of water by dehydroxylation should lead to the creation of one Lewis acid site from every two Brpnsted sites. 

Even when it can be demonstrated that binding results from proton transfer (adsorption at Brpnsted sites on the surface of the solid), the heat of adsorption is not a measurement of the proton affinity of the site. It is, in fact, a convolution of the proton affinity of the acid site on the solid, the proton affinity of the reference base, and the heat of interaction of the resnlting ion pair. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

It is known that the activation temperature can influence the acid strength distribution. For example, measurements of the differential heats of ammonia adsorbed at 150°C for a HY zeolite have led to the conclusion that stronger acid sites, in the 150-180 kJ/mol range, are formed upon increasing the activation temperature from 300 to 650°C. Dehydroxylation at high temperature resulted in the formation of strong Lewis acid sites and the disappearance of intermediate and weak Brpnsted sites [62]. 

The effect of temperature on ammonia adsorption by ZSM5 samples has been investigated by microcalorimetry, varying the adsorption temperature from 150 to 400°C [235]. The initial heats of adsorption were independent of temperature up to 300°C. When the adsorption temperature increased, there was a competition between the formation of ammonium ions on Brpnsted sites and their decomposition. The total number of titrated sites decreased with increasing adsorption temperature. It appeared that an adsorption temperature between 150 and 300°C is appropriate for these calorimetric experiments. 

The NMR observable most commonly exploited in studies of solid acidity is the chemical shift. While some NMR observables (e.g., dipolar couplings) lend themselves to a more or less direct quantitative evaluation, the chemical shift must be interpreted. Changes in the 13C or 15N isotropic shifts of adsorbates are observed upon complexation with Brpnsted sites, and the same is true of the H shift of the Brpnsted site, but one is hard pressed to interpret such changes quantitatively in terms of a detailed structure of the adsorption complex or even the extent of proton transfer. 

Fig. 1. 300-MHz H MAS spectra of zeolite HZSM-5 over a range of temperatures. In addition to the well-known Brpnsted sites (4.3 ppm) and external silanols (2.0 ppm), a broad shoulder at 296 K sharpened to a third peak at 6.9 ppm when the sample temperature was reduced to 123 K The spinning speed was 3.5 kHz. (Reprinted with permission from Beck et al. (24). Copyright 1995 American Chemical Society.)...

Figure 7 shows DFT calculations (BLYP/DNP) of the geometries of acetone and mesityl oxide on a cluster model of the Brpnsted site in zeolite... 

Our study of propene-J-13C, -2-13C, and -5-13C reacting on zeolite HZSM-5 clearly shows that the isopropyl cation is not formed in measurable concentration as a persistent species (45). Furthermore, there is no label scrambling of the 2 position, although 1,3-label scrambling is facile on the zeolite. This strongly argues against a free isopropyl cation—even as a transient intermediate At low temperature, the equilibrium structure of propene is a 77 complex 22 with the Brpnsted site. This mode of coordination... 

Do these results also suggest that five-coordinate carbonium ions are not essential to explain alkane cracking The evidence is mixed. Kazansky and van Santen (132) reported low-level calculations and found a metastable carbonium ion (CH3-H-CH1) formed from ethane and a zeolite Brpnsted site, but this species was so high in energy that it did not appear to be thermally accessible. More extensive work by van Santen (133) shows, however, that the transition states leading from this species do not relate to ethane cracking Blaszkowski, Nascimento, and van Santen (134) found other transition states for ethane cracking (Fig. 26) that are similar to carbenium ions albeit with stabilization from the lattice. 

Fig. 28. 36-MHz l5N CP/MAS spectra of pyridine-15N on zeolite HY. The experimental conditions were all the same for (a) and (b), except that sample b was extensively dealuminated by increasing the activation temperature to SSITC (400°C for sample a). Both spectra were acquired at 77 K to prevent chemical exchanges on the NMR time scale, (a) The single resonance at —176 ppm as well as its associated sidebands indicates protonation of pyridine by the Brpnsted sites, (b) In addition to the protonated pyridine, four additional resonances at -68, -88, —116, and -140 ppm are also seen, indicating complexation of pyridine with different extraframework Lewis sites.

Fig. 34. 31P MAS spectra of trimethylphosphine on zeolite HY. Both proton-decoupled (a) and nondecoupled (b) spectra are shown. The nondecoupled spectrum clearly shows the scalar coupling of 13P- H ( 7p H = 550 Hz), providing unambiguous evidence for the protonation of trimethylphosphine by the Brpnsted site. (Reprinted with permission from Rothwell et al (173). Copyright 1984 American Chemical Society.)...

That is, the Brpnsted acid site can be thought of as a hydrated Lewis acid site. Since the Brpnsted site can donate a proton to another species, we can represent its reaction with ammonia simply as the interaction of a proton with the unshared electron pair on the nitrogen atom of the ammonia molecule ... 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

Solid superacids can be further divided into various groups depending on the nature of the acid sites. The acidity may be a property of the solid as part of its chemical structure (possessing Lewis or Brpnsted sites the acidity of the latter can be further enhanced by complexing with Lewis acids). Solid superacids can also be obtained by deposition on or intercalation of strong acids into an otherwise inert or low-acidity support. 

Reaction mechanism It is generally admitted that, over zeolites, acetylation of arenes with AA is catalysed by protonic acid sites. Comparison of the activity of a series of dealuminated HBEA samples allows one to exclude any direct participation of Lewis acid sites in 2-MN acetylation with AA. Indeed, two HBEA samples with similar protonic acidities but with very different concentrations of Lewis acid sites (170 and 16 pmol g ) have practically the same acylating activity.1271 The role of Brpnsted sites is also clearly expressed in Spagnol et a/.131... 

In the case of the ortho to me/a-xylene methyl shift isomerization transition states, the steric constraints are less important as the nonparticipating methyl group has more available space because of the ellipsoidal shape of the 12-membered ring channel (see Figure 15). Then, the activation energies are + 168 kJ/mol, and + 184 kJ/mol for the transition state which follows immediately the xylene protonation, and for the transition state which occurs after xylene overcame a rotation energy barrier to change its orientation with respect to the Brpnsted site respectively. 

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